Phase change inks (sometimes referred to as “solid inks” and “hot melt inks”) have been used in various liquid deposition techniques. Phase change inks often contain a “phase change agent” that enables the ink to exist in a solid phase at ambient temperatures, but also exist in the liquid phase at the elevated operating temperature of an ink jet printing device. At the deposit operating temperature, droplets of liquid ink are ejected from the printing device and, as the ink is jetted towards or contacts the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, the ink quickly solidifies to form a predetermined pattern of solidified ink drops. Phase change inks have also been used in other printing technologies, such as gravure printing, as disclosed in, for example, U.S. Pat. No. 5,496,879 the disclosure which is incorporated herein by reference.
Phase change inks are desirable for ink jet printers because they remain in a solid phase at room temperature during shipping, long term storage, and the like. In addition, the problems associated with nozzle clogging as a result of ink evaporation with liquid ink jet inks are largely eliminated, thereby improving the reliability of the ink jet printing. Further, in phase change ink jet printers wherein the ink droplets are applied directly onto the final recording substrate (for example, paper, transparency material, and the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference, ink jet printing systems generally are of two types: continuous stream and drop-on-demand.
In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
There are at least three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink, filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (that is, liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In general, phase change inks are in the solid phase at, for example, ambient or room temperature, such as about 20° C. to about 27° C., but exist in the liquid phase at the elevated operating temperature of an ink jet printing device. At the jet operating temperature, the ink is molten and droplets of liquid ink are ejected from the printing device.
In a typical design of a piezoelectric ink jet device utilizing phase change inks printing directly on a substrate or on an intermediate transfer member, such as the one described in U.S. Pat. No. 5,372,852, incorporated herein by reference, the image is applied by jetting appropriately colored inks during four to eighteen rotations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head, that is, there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration.
Hot melt inks typically used with ink jet printers have a wax based ink vehicle, for example, a crystalline wax. Such solid ink jet inks provide vivid color images. In typical systems, these crystalline wax inks partially cool on an intermediate transfer member and are then pressed into the image receiving medium such as paper. Transfuse spreads the image droplet, providing a richer color and lower pile height. The low flow of the solid ink also prevents show through on the paper.
However, the use of crystalline waxes places limitations on the printed image. Conventional crystalline waxes are apolar aliphatic molecules bound together by weak van der Waals forces. These waxes have little natural affinity for the more polar paper substrate and due to the nature of their own intermolecular bonding are vulnerable to mechanical damage.
U.S. Pat. No. 6,906,118, incorporated herein by reference in its entirety, discloses phase change ink compositions wherein at a first temperature, hydrogen bonds of sufficient strength exist between the ink vehicle molecules so that the ink vehicle forms hydrogen bonded dimers, oligomers, or polymers, and wherein at a second temperature higher than the first temperature, the hydrogen bonds between the ink vehicle molecules are sufficiently broken that fewer hydrogen-bonded dimers, oligomers, or polymers are present in the ink so that the viscosity of the ink at the second temperature is lower than the viscosity of the ink at the first temperature.
Solid inks are considered to be an environmentally-conscious way to print due to the cartridge free design, minimal packaging, nearly waste-free printing process and safe, toxin free solid ink sticks. However, solid inks may also cause certain problems such as (1) increased accumulation in greenhouse gases and/or accumulation of non-biodegradable materials and (2) ink shrinkage (the purging of excess ink) caused by formation of air bubbles jet lines. These issues may be due to presence of petroleum-based products such as waxes in the solid ink, which can lead to poor abrasian resistance, poor adhesion and poor fold resistance. While present phase change inks are very useful for their intended purposes, it is desirable to provide a solid phase change ink wherein a major proportion of the ink is derived from renewable resources, and is thus environmentally friendly.